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		<H2><A NAME="HDRDATMOD" ></A>Chapter 3. Understanding the Data Model
</H2>
		
<P><A NAME="PToC6" HREF="../usrguide.htm">Partial Table-of-Contents</A> 
<MENU>
<LI><A NAME="PToC_50" HREF="#Header_50">3.1 Introduction to the Data Model</A>
<LI><A NAME="PToC_51" HREF="usrgu025.htm#Header_51">3.2 Object Types</A>
<MENU>
<LI><A NAME="PToC_52" HREF="usrgu025.htm#Header_52">Fields</A>
<LI><A NAME="PToC_83" HREF="usrgu025.htm#HDRARRAYS">Arrays</A>
<LI><A NAME="PToC_90" HREF="usrgu025.htm#Header_90">Groups</A>
</MENU></MENU><HR><P>
<A NAME="IDX484"></A>
<A NAME="IDX485"></A>
<A NAME="IDX486"></A>
<P>
This chapter describes the concepts and terminology of the data
model for data stored in the Data Explorer system, whether in memory
or on disk.
<P>
A complete understanding of this chapter is not required for the
effective use of Data Explorer, and the brief discussion of a Field
in <A HREF="usrgu022.htm#HDRVIS">Chapter 2. "Introduction to Visualization"</A>
should get you started.
However, the more detailed information here is useful when you have
specific questions about the data model.
<HR>
<H2><A NAME="Header_50" ></A>3.1 Introduction to the Data Model
</H2>
<P>
The Data Explorer data model supports various types of simulation
and observational data.
Data structures that can be represented include:
<UL COMPACT>
<A NAME="IDX487"></A>
<LI>Data defined on a regular orthogonal grid
<LI>Data defined on a deformed regular or curvilinear grid
<LI>Data defined on various irregular grids,
such as triangular, quadrilateral, and tetrahedral meshes
<LI>Unstructured data with no connections between the data samples.
</UL>
The data samples can be defined over spaces of any dimensionality, and,
independently, can also be connected by primitives of various
dimensionalities (allowing, for example, triangular and
quadrilateral meshes defined over 2- or
3-dimensional points).
The data values can be associated either with the sample points or with
the connections between the sample points.
Available data types include:
<UL COMPACT>
<LI>Real and complex data
<LI>Scalar, vector and tensor data
<LI>Byte, short, integer (signed and unsigned), and floating-point data
</UL>
<P>
<A NAME="IDX488"></A>
Data are stored in the form of <I>Objects</I> for use by Data Explorer
modules.
An Object is a data structure stored in memory that contains an
indication of the Object&#39;s type, along with additional
type-dependent information.
The bulk of the data is encapsulated in <I>Array</I> Objects.
<P>
The data model centers on the notion of a <I>sampled field</I>.
The next section describes the <I>Field, Array,</I> and
<I>Group</I> Objects that implement sampled  fields
in Data Explorer.
In addition to these basic Object types, other types are used to
construct models for rendering (e.g., Transforms, Clipped
Objects, Lights, and Cameras).
These are described in <A HREF="usrgu068.htm#HDREDF">B.2 , "Data Explorer Native
Files"</A> and in <I>IBM Visualization Data Explorer Programmer&#39;s
Reference</I>.
<P>
Data are also stored in permanent file storage in the form of the same
Objects.
Although Data Explorer supports the creation of Objects from data stored in
other file formats (such as netCDF), the Data Explorer file format offers
significant additional functionality and flexibility.
<P>
Note that the Data Explorer file format is versatile, allowing for future
expansion of the capabilities of the system without requiring
changes to the file format.
It is possible to represent data types in a Data Explorer file that cannot be
processed by the current version of Data Explorer.
For example, in the current release of Data Explorer, only single-precision
floating-point positions are universally supported.
Also, most modules support only 1-, 2-, or 3-dimensional positions.

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